1. Field of the Invention
The present invention relates to a power conversion system.
2. Description of the Related Art
FIG. 25 shows a circuit diagram of a power conversion system for cars like a converter circuit and an inverter circuit using semiconductor switching devices, for instance, GTO so far available and a block diagram of the system is shown in FIG. 26. A conventional power conversion system for cars is connected with a snubber circuit comprising a snubber diode 2, snubber condenser or capacitor 3, and a snubber resistor 4 at both ends of a semiconductor device 1. The snubber diode 2 and the snubber capacitor 3 are connected in series and both ends of these are connected in parallel to both ends of the semiconductor device 1. The snubber resistor 4 is connected to the snubber diode 2 in parallel.
This snubber circuit is for suppressing voltage applied to both ends of the semiconductor device 1 when it is switched within its rated voltage and it is needed to reduce the inductance of a looped circuit comprising the semiconductor device 1, snubber diode 2 and snubber capacitor 3. Therefore, it becomes necessary to arrange the snubber diode 2 and the snubber capacitor 3 immediately close to the semiconductor device 1. The snubber resistor 4 is provided for the purpose of discharging energy accumulated in the snubber capacitor 3 and is arranged at a portion (an opening portion) of the system to communicate with the atmosphere likewise cooling fins of a semiconductor cooler 5 in many cases by considering heat radiation.
A switching command is output from a gate amplifier 6 and input to the semiconductor device 1. Further, for the power for a main circuit of a power converter, power of stabilized voltage is supplied from a filter capacitor 7 connected in parallel to a power converter. The filter condenser or capacitor 7 is connected to a power source outside the system.
In case of a conventional power conversion system for use in railway cars in the above-mentioned structure, it was needed to arrange two kinds of parts, that is, the snubber diode 2 and the snubber capacitor 3 immediately close to the semiconductor device 1 and by bringing the terminal positions of these parts close to each other so as to reduce inductance of connected wires as could as possible. However, in order to arrange two kinds of different parts closely each other, special shaped parts had to adopt and security of reliability and reduction in part costs using standard shaped parts were difficult and furthermore, the narrowed spaces between parts impeded the flow of heat generated inside the system, and a problem of temperature rise in the system was generated. Further, regarding the construction, the system was not rationally assembled and if troubles occurred, the inspection and removal of parts were difficult.
In addition, as heat to be radiated to the outside of the system, it is necessary to radiate not only heat generated from the semiconductor device 1 but also heat generated from the snubber resistor 4 and for instance, in case of a power conversion system that is installed under the floor of a railway carriage, the temperature under the floor is raised by the exhaust heat from the system and adversely affected the feeder line under the floor not a little. Therefore, it was so far demanded to realize a power conversion system that is highly reliable, small in size, light in weight, simple in construction and capable of suppressing a price low.
However, in order to realize such a power conversion system with a snubber circuit eliminated, it was necessary to solve technical problems shown below. In the main circuit for electric car driving, surge voltage Vp is generated when large current I of 2300A is cut off even when DC voltage is 1800V as shown in the chart in FIG. 27. So, for such a power transistor as GTO that is adopted in an inverter circuit and a converter circuit as a power conversion system, a power transistor of a lower rated value that suppresses the peak of surge voltage Vp by absorbing surge voltage by providing a snubber circuit was so far adopted.
However, in case of such a voltage driving type high-frequency switching device as IGBT (Insulated Gate Bipolar Transistor) or IEGT (Injection Enhanced Gate Transistor) that is capable of high-speed switching at continuous 700 Hz (a conventional GTO is a current driving type switching device capable of switching at about continuous 500 Hz), when inductance of a loop circuit enclosed by conductors connecting a switching device and a capacitor is reduced by reducing internal inductance of a surge voltage absorbing capacitor and that of conductors as could as possible, it becomes possible to construct a semiconductor switching circuit requiring no snubber circuit.
In other words, as shown in FIG. 28, DC voltage for a main circuit for an electric car is generally 1800V and when a semiconductor switching device of rated voltage 3300V is adopted and the system is so set as to suppress surge voltage Vp generated when breaking the semiconductor switching circuit by breaking current 2300A to suppress to below 3000V with a margin of about 10% against the rated voltage of the switching device, it becomes possible to compose a power converter with less a snubber circuit when an inductance of the loop circuit is suppressed to 250 nH or below.
In order to suppress an inductance of the loop circuit to below 250 nH, when a capacitor having an internal low inductance of about 50 nH is adopted and a circuit is in a physical structure to suppress an overall inductance of conductors to below 200 nH, it is possible to realize a circuit satisfying the condition. So, in order to realize such a circuit, it is needed to bring surge voltage absorbing capacitors and semiconductor switching devices as close as possible and also, bring conductors connecting semiconductor devices each other and conductors connecting semiconductor switching devices and capacitors so that an area enclosing a loop circuit is made small and lengths of conductors are made short.